This invention relates to a fluidic flowmeter for measuring the flowrates of natural gas (city gas) and other gases.
The fluidic flowmeter is a flowmeter for measuring the flowrate by structuring an adhesion wall and a feed-back flow channel on a down-stream side of a nozzle to blow out the measured fluid and to generate a fluid vibration relative to the adhesion wall by use of the Coanda effect wherein the pressure propagation is transmitted to the feed back flow channel, and by designing the fluid vibration (oscillation frequency) to be proportional to the measured fluid.
As a fluidic flowmeter with this type of structure, the flowmeters stated in U.S. Pat. No. 3,640,133, U.S. Pat. No. 3,690,171, Japan Patent Official Disclosure No. 48-54962, Japan Patent Official Disclosure No. 53-77558, Japan Patent Official Disclosure No. 59-184822, etc. are already known to the public.
The aforementioned fluidic flowmeter is of a system for generating a fluid vibration inside the fluidic element and electrically using this vibration for controlling the flowrate of the measured fluid while using a micro computer. There appears no problem if the flowrate is comparatively large. However, the generation of the fluid vibration becomes unstable at a very low flowrate, causing a measurement error.
Therefore, it has been proposed to selectively measure the flowrate higher than a constant flowrate by use of a fluidic element and measure the flowrate lower than a constant flowrate by use of a flow sensor inserted into the nozzle portion. (Japan Patent Disclosure No. 1-58,118).
However, in the case of the aforementioned officially known examples, if a cable disconnection or a contact defect should happen in the flow sensor and the sensor for detecting the fluid vibration, generated by the fluidic element, and in the lead wire connecting these two sensors with the micro computer for carrying out the operation and the like, there may be a case of measuring the flowrate without noticing the disconnection problem.
Secondly, because the micro flow sensor has a high stability, it is generally used without performing the zero point correction. However, because a slight deviation from zero point is contemplated during manufacture over time, conventionally a method has been adopted including installing a dead band against the output signal in the vicinity of zero flowrate for disregarding, namely, not integrating the output in this range.
However, in the aforesaid system of disregarding the output in the vicinity of zero flowrate, no correction is made at all regardless of a drift of the zero point due to adjustment error during manufacture and fluctuations over time in the case that small pulse outputs lower than the necessary minimum detected flowrate occupy almost all the outputs, and for this reason, the drift error in the flow sensor measuring range cannot be removed. Especially in the small flowrate region, the size of such zero drift has a big influence on the measurement accuracy since it has a comparatively great value against the flowrate signal output.
Next, the micro flow sensor has a high durability and its performance rarely changes, but in preparation for the case that its sensibility changes because of the adhesion of some amount of dust and the like as many years go by, a so-called gain correction function for automatically building up the sensibility of the flow sensor has been incorporated conventionally into this type of flowmeter.
The conventional gain correction provides a system of automatically building up the sensibility of the flow sensor on the basis of the measurement flowrate of the fluidic element in the region where both the flow sensor and fluidic element work, and is based on a calculation system as given below.
When the pulse output of the flow sensor is P, there exists the following relation to the flowrate indicated value Q.sub.(FS) by the flow sensor. EQU Q.sub.(FS) =K.sup.i.sub.(FS) P. (1).
where, K.sup.i.sub.(FS) is the flow sensor gain at the point of time i.
Now, suppose the gas flowrate has entered the region such that it has been measured with both the flow sensor and fluidic element. ##EQU1## where, P.sup.i is the mean value of the number of pulses of the flow sensor during its correction, while the Q.sup.i.sub.(FD) is the mean value of flowrate measured by the fluidic element. In the conventional method, the correction shall be made by using the value for K.sup.i obtained by Formula (2) as a new flow sensor gain. That is to say, EQU K.sup.i+1.sub.(FS) =K.sup.i ( 3)
where the K.sup.i+1.sub.(FS) is the flow sensor gain at the point of time i+1.
In the this conventional example, though the measured values of flow sensor and fluidic element indicate a high accuracy as the timely averaged values, the values at every measurement entail the fluctuations due to the flow disturbance and the noise. Therefore, to correct the flow sensor with a high accuracy, the outputs of fluidic element and flow sensor need to be measured over many hours for averaging the values. However in the case of the gas meter and the like that are installed at the end-user household, it is impossible to flow the gas for test purpose when correcting the flowrate, and the flowrate needs to be corrected while the gas is used by the end-user household, so the longer period required for correcting the flowrate results in the fall of correction frequencies. In addition, even in case that noise gets mingled with the measured value during the correction for some reason or other, no function for confirming it is available, and hence there is a possibility for the error in flow sensor region to increase temporarily.